Method of making fine-grained steel



AVERAGE GRAIN SIZE, ASTM N0.

April 15, 1969 R. M. ALLEN 3,438,822

METHOD OF MAKING FINE-GRAINED STEEL F iled 00, 51. 1966 7 I I 1 I I I I I I I I l I 1 I 0.! 2 4 6 8 I0 20 40 6'0 30 I00 HEAT/N6 RATE FROM /300F. T0 AUSTEN/T/Z/NG TEMR, F. PER SE6.

LEGEND -l/Z INCH THICK PLATE 3 INCH ANGLE INVENTO/i. ROBERT M. ALLEN Ar rarney United States Patent US. Cl. 148-143 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of improving the notch toughness of steel. More particularly, the invention relates to an improvement in the method of producing austenitized quenched and tempered steel sections of a particular composition to obtain a fine-grained metallurgical structure having an average grain size of ASTM No. 10 or finer. A method is disclosed herein for treating steel of a particular composition to produce a fine-grained structure and data is presented to show criticality of certain processing conditions to this end. The steel treated has the following composition: up to 0.18%, 0.1 to 0.4% manganese, 0.15 to 0.35% silicon, 2.0 to 2.5% nickel, 1.0 to 1.8% chromium, 0.2 to 0.4% molybdenum, up to 0.025% phosphorus, up to 0.025% sulfur and up to 0.25% copper. The treatment comprises an improvement in the method of making austenitized, quenched and tempered steel sections up to about l-inch thick of the aforementioned composition wherein the heating from 1300 F. to austenitizing temperature is performed at a rate between 2 and 20 F. per second. Steel of the composition described above responds in an unusual manner to dilferent heating rates. As is shown by data presented herein, the grain size first decreases and then increases as the heating rate gets faster. When the rate is within the above limits, the desired fine-grain size may be obtained.

The influence of alloying elements upon the notch toughness of steels is well known. It is also known that rapidly heating the steel from the A temperature to austenite results in an improvement in mechanical properties. I have discovered, however, that with a steel within a particular composition known commercially as Hy-SO and of limited section thicknesses, the response to the rate of heating to austenitizing temperature varies in an unusual manner such that the grain size begins decreasing between the heating rate of 5 and 6 F. per second as the steel is heated in the range of 1300 F. to the austenitizing temperature. Conventional heat treating of austenitized quenched and tempered steel sections is conducted at rates well below 2 per second.

At average grain sizes of ASTM No. 10 or finer, the mechanical properties are generally superior than coarser grained structures and accordingly, it is desirable to produce austenitized quenched and tempered steel having an average grain size of ASTM No. 10 and finer. I have discovered that when steel of the aforementioned type is austenitized by heating to austenitizing temperature at rates of 2 to F. per second through the temperature range of 1300 F. to the austenitizing temperature (usual- 1y about 1600-1700 F.) the desired fine-grained structure may be obtained. Moreover, steels so produced will 1 have a 30 ft./lb. Charpy V-notch transition temperature of less than F. and a yield strength of greater than about 140 K s.i.

The invention will be more readily understood by the ensuing discussion and the accompanying drawing which is a graph illustrating the variation in grain size with heating for steel within the purview of the invention. As can be seen from the heating rate grain size logarithmic curve shown in the drawing, the upper limit of heating rate, in the range 1300 F. to the austenitizing temperature, necessary to obtain a grain size of ASTM No. 10 or finer is just below 20 F. per second. This curve also shows that the range of heating rate in the steel of the aforementioned composition extends from just above 2 per second to just below 20 per second. This response to the heating rate seems to be peculiar to this steel composition and I know of no other austenitized, quenched and tempered alloy steels that undergo the same response of grain size variation with austenitizing heating rate.

Compositions within the ranges described in Table I were found to be responsive to the rapid austenitizing treatment which comprises the improvement in accordance with the invention.

TABLE I [Composition Range of I-Iy-SO Steel for Rapid Heat Treat- 1ng-Percent by Weight] The yield strength and notch toughness data for samples within the composition range described in Table I but of the specific compositions shown in Table II were prepared in plates and angles of stated thickness and austenitized at heating rates varying from 0.8 to 79 F. per second in the critical range 1300" F. to the austenitizing temperature. The average grain size, yield strength and notch toughness is described in Table III. The strength and toughness data indicates that the steels exhibit a 30 ft./ lb. Charpy V-noteh transition temperature of -70 F. to as low as -l F. for the yield strength range of 141 to 158 K s.i. It should be noted that with steels austenitized at rates slower or faster than the 2 to 20 F. per second range exhibited 30 ft./lb. transition temperature of 30 to 40 F. for a similar yield strength range (143 to 157 K s.i.

TABLE IIL-EFFECT OF HEATING RATE ON THE YIELD STRENGTH AND NOTOH TOUGHNESS Heating Rate Yield Charpy V-Notch (1,300+ F. to Average Strength 30-it./lb. Transition Product Heat Treated Austenitizing Grain Size, 0.2% Ofiset, Temperature,

Temperature) ASTM N 0. K s.i. FA

F. per See.

A Plate in. thick 0.8 9.0 157 30 (/2, 0.8 9. 2 151 35 2. 9. 8 151 35 3. 0 10. 8 145 120 3. 0 11. 0 150 115 10 10. 156 90 10. 5 158 -70 10 10. 5 150 -1G() 10 10.8 145 -120 10 11. 0 143 100 21 9. 8 152 40 79 7. 3 143 30 B Angle (3 by 3-in.) 1. 5 9. 5 153 40 3. 0 10. 0 141 120 8. O 10. 0 143 130 1 After water quenched and tempered at 9001,200 F.

The range of heating rates of just above 2 F. per second to just below F. per second are applicable to steels of the aforementioned composition in section thickness of from -inch to l-inch. Obviously, eifectiveness of the heating rate on the steel stock is dependent to a large degree on size of the section. For many constructional applications, section thickness of to l-inch are required and within this range it has been determined that the best combination of properties can be obtained when the critical heat treating rate during austenitizing is observed.

The thickness of sections that can be heated using rapid heat treating cycles with fast heating rates and short holding times is limited by the allowable maximum surface to center differential that can be tolerated without impairing the desired metallurgical or mechanical properties and the power density limitations of the heating time. Extensive investigations have shown that for austentized quenched and tempered steel sections of section thickness of from about to l-inch of steels of the aforementioned composition desired metallurgical structure and fine grain size, i.e. ASTM No. 10 or finer, can be achieved if the heating rate is maintained within the critical range of 2 to 20 F. per second throughout the temperature range of 1300 F. to the austenitizing temperature. Products so obtained will have a 30 ft./1b. Charpy V-notch transition temperature within about 70 F. with a yield strength of at least about 140 K s.i.

I claim:

1. In the method of producing austenitized, quenched and tempered steel sections of a section thickness of up to about l-inch and wherein said steel consists essentially of, in percent by weight, up to 0.18% C, 0.1 to 0.4%

Mn, 0.15 to 0.35% Si, 2.0 to 2.5% Ni, 1.0 to 1.8% Cr, 02 to 0.4% Mo, up to 0.025% P, up to 0.025 S, up to 0.25% Cu, the improvement which comprises heating said steel to austenitizing temperature so that the heating rate from 1300" F. to the austenitizing temperaure is between 2 to 20 F. per second to obtain a metallurgical structure having an average grain size of ASTM No. 10 or finer.

2. An improvement according to claim 1 wherein the austenitized product has a metallurgical structure with average grain size of ASTM No. 10 or finer, and a C 30 ft./lb. transition temperature of less than F. at a yield strength of at least K s.i. after tempering.

3. An improvement in the process according to claim 1 wherein said heating rate is at least 5 F. per second.

References Cited UNITED STATES PATENTS 6/1957 Korczynsky 148-134 X 8/1966 Steiner 148-36 X OTHER REFERENCES CHARLES N. LOVELL, Primary Examiner.

U.S. Cl. X.R. l4836, 134 

